Implement :only to be the same as :tabonly for now

As discussed at #1168, this implements `:only` to be equivalent to `:tabonly`, but in a way that makes it easy to change this decision later. Specifically in the "pure Vim" parts of the extension, it is implemented as if they are different, but in the VS-specific parts, they are implemented to be equivalent.

This is not implemented in the Vim App at this time, though I have been looking into that.

This fixes #1168.